In general, closure fastening devices for use in connection with plastic bags and the like are known. Furthermore, manufacturing methods for closure fastening devices made of plastic material are generally well-known.
In operation, a closure fastening device for use in connection with a flexible container should be relatively easy to open from the outside, but relatively difficult to open from the inside. Generally, such a container can be used with its interior either under relatively high pressure or under relatively low pressure with respect to ambient conditions. The closure fastening device should provide a satisfactory seal for either condition.
Preferably, the closure fastening device should be suitable for economical manufacturing and should be relatively simple in design. In addition, the design should provide for variations in order to meet different needs. For example, it may be desirable to have a closure fastening device which is relatively difficult to open both from the inside and the outside. In general, the closure fastening device, however, should always be relatively easy to close.
In addition, when the closure fastening device is employed with a container, the container may be made from a thermoplastic material, and the closure device and sidewalls of the container can be made integrally by extrusion as a unitary piece, or can be made as separate components which are subsequently permanently connected together.
However, the thermoplastic resin materials heretofore found practical for the extrusion of interlocking closure devices, and their attachment to films, such as in making containers, have resulted in shrinkage and distortion problems during their use at elevated temperatures. Typical resin materials employed for interlocking closure devices and container films have included polyethylenes, polyvinyl chloride copolymers, and synthetic rubbers. However, none of these construction materials have sufficient thermal tolerance for many commercial uses. Further, both occlusion and deocclusion of the prior art interlocking closure devices are generally difficult to accomplish by the user when the device is made from resin materials having high temperature tolerances due to the higher flexural moduli usually associated with resins having higher temperature softening points.
The rapid advent of the working housewife, currently comprising about fifty percent of all households, has brought with it the need for time-saving and labor-saving devices. More then ever, householders perpare meals in advance and freeze them, as well as cook larger portions than required for a single meal. Quick cooking appliances like microwave ovens are rapidly increasing their market share and, not surprisingly, labor saving devices, even disposable devices, are finging increasing use.
Containers of the type considered herein have wide consumer use and usually feature two flexible side walls and a closure fastening device which can generally withstand moderate forces which would tend to open the container unexpectedly due to internal pressure. One more recent use of such containers is in microwave cooking of foods packaged therein. Thus, foods packaged in such containers may be stored in a freezer, removed therefrom, and placed in a microwave oven, where the foods are cooked directly in the containers. Likewise, foods packaged in such containers may be taken from a freezer and placed in boiling water to cook the foods.
However, such food storage bags and cooling containers, when made from thermoplastic resin materials, must meet stringent requirements. For example, when the food container is placed in boiling water, temperatures of up to about 215.degree. F. may be reached, and on a gas range or electric stove, temperatures may reach up to about 320.degree. F. above the water level on the wall of a skillet. Likewise, the fat content of meats may easily reach temperatures of about 300.degree. F. in a microwave oven.
Unfortunately, it has been found that conventional food containers made from thermoplastic resins such as polyvinylidene chloride and polypropylene develop leak holes, and that food containers made from polyethylene are severely damaged, unless the resin structures are very thick, when they are employed at cooking temperatures of about 300.degree. F. Thus, it would be desirable to provide a food container that could be used as a food storage bag, and that could also withstand thermal abuse, by providing thermoplastic resin containers capable of withstanding temperatures of about 280.degree. F. on the inside of the container, and temperatures of more than about 350.degree. F. on the outside of the container.
Another requirement for in-home use of such food containers is the capability of expelling air therefrom prior to closing them for the storage and preservation of foods. Typically, the expulsion of air from a food container would involve opening a small, i.e., about one-fourth to one inch, segment of the closure fastening device without the fastening device spontaneously completely deoccluding. However, some conventional closure devices do not possess such a controlled deocclusion or separation characteristic of the closure elements so as to enable the user to only partially open the container. A further requirement of thermoplastic containers used for cooking food is that the inside or pouch facing portion of the closure device be able to withstand much higher inflation forces than normally expected due to the development of internal pressure (such as by air expansion or steam generation), during the cooking of foods. An additional requirement of such food containers is that the thermoplastic material employed to make the closure device be compatible with the walls of the pouch to permit joining the closure device to the polymeric materials of the pouch walls or container sidewalls.